1. Field of the Invention
This invention relates to the use of digital correlation receivers to measure the parameters of non-cooperative coherent or non-coherent emitters.
2. Brief Description of the Prior Art
Prior art approaches for measurement of relative phase, frequency and pulse repetition interval (PRI) for coherent emitters and the measurement of relative phase and frequency for a non-coherent emitter at the output of a correlation receiver use predominantly time domain techniques to characterize non-coherent, pulsed and asynchronous signals. Coherent processing in pulse doppler radar systems uses either time or frequency domain techniques to determine doppler shift with respect to a known frequency reference. Pulse compression in radar systems by both compressive and transform techniques achieves correlation with a known reference waveform. Compressive receivers and scanning receivers analyze the spectra of pulsed waveforms in receivers and spectrum analyzers. Fourier transform based receivers analyze frequency domain spectra. Correlation receivers in both time domain and frequency domain architectures measure time difference of arrival and differential doppler. Channelized receivers employ brute force frequency domain techniques to separate simultaneous signals by using a parallel bank of filters that provide frequency selectivity. If two signals are simultaneously present in a single channel, the channelizer may indicate the presence of multiple signals, but cannot measure truly simultaneous signals under all conditions of signal amplitude and relative time delay. An example of this is the presence of synchronized or nearly synchronized signals with harmonically related PRI values. For this condition, the channelizer will frequently fail to measure the parameters of either or both signals.
Instantaneous frequency measurement (IFM) receivers cannot measure the frequency of simultaneous signals. The IFM indicates the frequency of the largest signal in a simultaneous signal interception. An IFM receiver requires about 10 dB signal-to-noise (SNR) ratio at an intermediate frequency (IF) to properly indicate frequency.
Compressive receivers provide selectivity and multiple signal handling capability, but do not allow accurate measurements of PRI. These receivers permit estimation of pulse width at high SNR values, but have limited dynamic range and limited time resolution. Compressive receivers have a fixed time-bandwidth product for each design realization. This limits the performance of compressive receivers in environments that have a variety of signals. Compressive receivers lose sensitivity (SNR) when the signal does not completely fill the time aperture of the compressive receiver. Sensitivity loss is a function of duty cycle and can be calculated from the equation: -20.star-solid.log (duty), where (duty) is the fractional ratio of the time the signal is present to the time aperture of the receiver.
Correlation receivers estimate time difference of arrival for signal detection in analog and digital realizations in a variety of applications. These receivers do not measure angle of arrival, PRI, pulse width or frequency in the digital domain using the methods described in the present application. If multiple signals are present in the same time aperture, the previously reported correlation processes cannot measure the signal parameters. The near zero delay terms of the correlation output contain information related to all the signals present in the receiver time aperture.
The performance of a time domain receiver is severely limited in the presence of multiple signals and in high density environments. This is because time domain receivers use some form of IFM to measure frequency and multiple time overlapping signals interfere with each other due to non-linear interactions within the IFM limiter. Typically, only one signal is measured at a time and the measurement is incorrect if two or more signals are present.
Fourier transform based receivers suffer sensitivity loss [-20.star-solid.log(duty)] for signals that do not fill the time aperture of the receiver. This results in unacceptable sensitivity for low duty cycle signals.
Scanning receivers used in spectrum analyzers and EW systems provide a low probability of signal intercept due to the narrow instantaneous bandwidth. Multiple scans must be used to characterize moderate to low duty cycle signals.